1. Field
This invention relates generally to pressure vessels, such as sterilizers. Specifically, this invention is directed to apparatus and methods for measuring the pressure within the chamber of a pressure vessel which compensate for pressure variations due to extant atmospheric pressure.
2. State of the Art
Pressure vessels, such as sterilizers, are used to treat various items with sterilants to disinfect them. Items which are typically subjected to sterilization in pressure vessels include dishware, surgical drapings, and surgical instruments Pressure vessels used for these purposes are well-known. Sterilizers generally comprise an outer housing which contains a chamber into which items for sterilization are placed, and structure for introducing sterilants into the chamber.
It is important for proper sterilization of items within the chamber to provide a sufficient amount of pressure within the chamber over a sustained period of time. Air is typically removed from the chamber of the sterilizer before the sterilant is introduced. If the air is not completely removed, it can affect the degree to which the sterilant penetrates the items placed within the chamber. Further, in gas sterilization, for example, the pressure in the chamber is used to determine how much sterilant has been introduced, and hence the concentration of the sterilant. It is important, therefore, to determine accurately the amount of pressure within the chamber at any given time, and to assure that pressure within the chamber has reached an appropriate level for thorough and effective sterilization.
Many sterilizers include structure for determining the amount of pressure within the chamber. Such structure typically includes a pressure transducer for determining pressure changes and levels. The transducer is commonly placed in direct communication with the sterilization chamber. Most pressure transducers used in connection with pressure vessels are vacuum referenced; that is, they are referenced at absolute pressure (zero pounds per square inch). These transducers produce a low level output signal proportional to the amount of pressure existing within the chamber. Such pressure measuring systems provide measurement of pressure within the chamber in terms of millivolts which are then amplified to volts. Because computers or microprocessors are typically used with such systems, the analog measurement must then be converted into a digital measurement by means of analog-to-digital converter apparatus. Pressure transducers typically used in sterilizers, in addition to being referenced to absolute zero, are uni-directional and provide measurement of increasing pressures only.
Accurate pressure measurements are seldom achieved in systems currently in use for two main reasons: (1) inaccuracies in measurement due to the physical and structural properties of the equipment used; and (2) inaccuracies due to the reference points used in evaluating the pressure at any given time. Some inaccuracies occur because of losses resulting from the low level signal produced by the transducer. Inaccuracies are also caused by noise in the cable connecting the transducer to the other structures of the component system, such as the amplifier and the analog-to-digital converter. When the millivolt signal is amplified, errors occur as a result of the inherent inaccuracies of the integrated circuitry in the amplifier, including tolerance anomalies and temperature drifting. Additional errors occur because of tolerance and temperature drifting in the components of the analog-to-digital converter. Resolution errors are also inherent in analog-to-digital converters.
Pressure measurements made by existing methods are also inaccurate due to the fact that most transducers are referenced to a zero pounds per square inch (0 psi) reference point. When the chamber is pressurized, it is impossible to determine accurately with current methods what part of the indicated increase in pressure is due to actual pressurization and what part is due to differences in atmospheric pressure at the location where measurement is taking place. Atmospheric pressure may vary from between about 14.7 psi at sea level to about 11.0 psi at higher elevations. The determination of accurate pressure within the chamber is significantly reduced with current systems since they lack any means of compensating for variations in ambient atmospheric pressure.
Thus, there exists a need for a pressure measurement system and method which reduces the errors experienced with current pressure measurement equipment and which further reduces error by compensation for existing atmospheric pressure variations.